1. Field of the Invention
The present invention relates to methods of etching silicon, and more particularly to methods of etching silicon while maintaining good photoresist selectivity.
2. Brief Description of the Background Art
Integrated circuit manufacturing processes often involve the creation of etch patterns in various materials by selective etching. For example, trenches are often made in a substrate such as silicon to provide isolation between individual devices or to provide capacitive charge storage or to define the gate for a transistor. Depending on the application, trench depths typically range from 1 to 4 microns in width and 0.5 to 5 in depth, although depths and widths beyond these ranges are clearly possible. In order to etch such silicon trenches, either a photoresist mask or an oxide hard-mask is typically used.
Commonly, these etch patterns are created by providing a patterned photoresist layer upon the material within which the etch pattern is to be made. Defining trenches with a photoresist mask simplifies the integration sequence and allows sidewall taper to be controlled. In general, the smaller the feature size that is required, the thinner the photoresist layer is required to be. Unfortunately, the thickness of a given photoresist layer frequently limits the thickness of the material that is to be etched, based on selectivity that exists between the photoresist and the material that is to be etched. Hence, by increasing selectivity, a thinner photoresist layer can be used.
In this connection, traditional silicon/polycrystalline silicon etching processes, and specifically polysilicon gate etching processes, are typically based on HBr/Cl2 chemistry, which has its limitations. For example, the silicon:resist selectivity with this chemistry has an upper limit of about 2.5:1. Moreover, this chemistry produces significant chamber deposition, since etch by products such as SiOx and Bry are non-volatile. Related chemistries, including HBr/Cl2O2 and CF4/HBr/Cl2/O2 chemistries, also have limitations, including silicon:resist selectivities of less than 3:1.
Other chemistries are known, such as SF6/HBr/O2 chemistry and SF6/CFH3/O2 chemistry, which have good etch rates (about 1 micron/min), but have relatively low silicon:resist selectivities (about 2-3:1) and produce re-entrant profiles with rough sidewalls. Other chemistries include a three-gas etch chemistry utilizing a CF4/Cl2/N2 combination, producing less than a 1.5:1 selectivity.
With the trend toward smaller feature sizes, (e.g., the use of 193 nm resist instead of the current deep ultraviolet resist techniques), resist selectivities and resist thicknesses are expected to decrease even further from those associated with 248 nm photoresist.